Methods and systems for variable displacement engine control

ABSTRACT

Methods and systems are provided for selecting a group of cylinders for selective deactivation, in a variable displacement engine system, based at least on a regeneration state of an exhaust catalyst. The position of one or more valves and throttles may be adjusted based on the selective deactivation to reduce back-flow through the disabled cylinders while also maintaining conditions of a downstream exhaust catalyst. Pre-ignition and knock detection windows and thresholds may also be adjusted based on the deactivation to improve the efficiency of knock and pre-ignition detection.

FIELD

The present application relates to methods and systems for controlling avariable displacement engine (VDE).

BACKGROUND AND SUMMARY

Engine exhaust catalysts require periodic regeneration to restorecatalytic activity and reduce catalyst oxidation. For example, catalystsmay be regenerated by rich injecting fuel to reduce the amount of oxygenstored at the catalyst. Catalyst regeneration strategies may vary fordifferent engine systems.

One example approach for exhaust catalyst regeneration in a variabledisplacement engine (VDE) system is described by Doering et al. in U.S.Pat. No. 7,044,885. Therein, the engine includes two cylinder groups,each coupled to respective catalysts. An engine control system isconfigured to alternate the cylinder group that is selectively disabledduring a VDE mode of operation to balance engine wear and catalystusage.

However, the inventors herein have identified a potential issue withsuch an approach. If the catalyst on one of the cylinder groups ispartially regenerated and the other cylinder group is selected fordisablement (based on the alternating pattern), fueling may be continuedto the partially regenerated catalyst, to complete regeneration, beforethe cylinders of that engine can be operated. Consequently, additionalfuel may be consumed to complete the regeneration. As such, this resultsin degraded fuel economy.

Thus, in one example, the above issue may be at least partly addressedby a method of operating an engine including a first group of cylinderscoupled to a first catalyst and a second group of cylinders coupled to asecond catalyst. In one embodiment, the method comprises, selectivelydeactivating either the first or the second group of cylinders, theselective deactivation based on a regeneration state of the firstcatalyst relative to the second catalyst. In this way, by selecting thegroup of cylinders with the less regenerated catalyst, additional fuelmay not be wasted in completing catalyst regeneration.

For example, a variable displacement engine (VDE) may be configured witha first and a second group of cylinders, each cylinder including aselectively deactivatable fuel injector. In response to a cylinderdeactivation request (such as, at low engine loads), an engine controlsystem may compare the regeneration state of a first catalyst, coupledto the first group of cylinders, relative to a second catalyst, coupledto the second group of cylinders. If the first catalyst is lessregenerated than the second catalyst, the first catalyst may be selectedfor deactivation. In comparison, if the second catalyst is lessregenerated than the first catalyst, the second catalyst may be selectedfor deactivation. In another embodiment, the selection may be furtherbased on a catalyst temperature. As such, if both the catalysts aresufficiently regenerated, the control system may select a cylinder groupbased on the deactivation order. Thus, if the first group of cylinderswas deactivated in the previous VDE cycle, the second group of cylindersmay be selected for deactivation on the current cycle, and vice versa.By alternating cylinder group deactivation when both catalysts aresubstantially regenerated, engine wear and tear may be balanced betweenthe two groups of cylinders.

In this way, by selecting a group of cylinders based on the regenerationstate of exhaust catalysts, fuel wastage can be reduced. Specifically,by selectively deactivating a group of cylinders that is coupled to apartially regenerated catalyst, even if the same group of cylinders waspreviously deactivated in a preceding VDE cycle, fuel loss related tocomplete regeneration of a catalyst (on a cylinder group that issubsequently deactivated), can be pre-empted. In this way, fuel economymay be improved without substantially affecting engine or catalyst wear.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example layout of a variable displacement engine system.

FIGS. 2A-C show alternate embodiments of the variable displacementengine system of FIG. 1 including a turbocharger.

FIG. 3 shows a partial engine view of the engine system of FIG. 1.

FIG. 4 shows a high level flow chart for operating the variabledisplacement engine system of FIG. 1.

FIG. 5 shows a high level flow chart for selecting a cylinder group fordeactivation.

FIGS. 6-7 show a high level flow chart for adjusting the position of oneor more valves and throttles of the engine system of FIG. 1 based on thedeactivation.

FIG. 8 shows a high level flow chart for detecting and differentiatingpre-ignition and/or knock.

DETAILED DESCRIPTION

The following description relates to systems and methods for adjustingcylinder operation in a boosted engine system (such as the VDE enginesystem of FIGS. 1-3). The engine system may switch between operationwith all cylinders firing or fewer cylinders firing by selectivelydeactivating one or more cylinder fuel injectors. An engine controllermay select a group of cylinders for deactivation based on a regenerationstate of a downstream catalyst (FIGS. 4-5). Based on the selection, theposition of one or more valves and throttles may be adjusted to maintaincatalyst temperature, reduce back-flow through the disabled group ofcylinders, improve boost operations, and coordinate EGR operations withVDE operations (FIGS. 6-7). The engine control system may also adjusteach of a window and a threshold for knock and pre-ignition detectionbased on the deactivation (FIG. 8) to improve detection and mitigationof abnormal cylinder combustion events. In this way, by adjustingvarious cylinder operations in a VDE system based on the deactivation,fuel economy and engine performance improvements can be achieved.

FIG. 1 shows an example variable displacement engine (VDE) 10 having afirst bank 15 a and a second bank 15 b. In the depicted example, engine10 is a V8 engine with the first and second banks each having fourcylinders. Engine 10 has an intake manifold 16, with throttle 20, and anexhaust manifold 18 coupled to an emission control system 30. Emissioncontrol system 30 includes one or more catalysts and air-fuel ratiosensors, such as described with regard to FIG. 2A. As one non-limitingexample, engine 10 can be included as part of a propulsion system for apassenger vehicle.

During selected conditions, such as when the full torque capability ofthe engine is not needed, one of a first or a second cylinder group maybe selected for deactivation (herein also referred to as a VDE mode ofoperation). Specifically, one or more cylinders of the selected group ofcylinders may be deactivated by shutting off respective fuel injectorswhile maintaining operation of the intake and exhaust valves such thatair may continue to be pumped through the cylinders. While fuelinjectors of the disabled cylinders are turned off, the remainingenabled cylinders continue to carry out combustion with fuel injectorsactive and operating. To meet the torque requirements, the engineproduces the same amount of torque on those cylinders for which theinjectors remain enabled. This requires higher manifold pressures,resulting in lowered pumping losses and increased engine efficiency.Also, the lower effective surface area (from only the enabled cylinders)exposed to combustion reduces engine heat losses, improving the thermalefficiency of the engine. In alternate examples, engine system 10 mayhave cylinders with selectively deactivatable intake and/or exhaustvalves.

Cylinders may be grouped for deactivation in a bank-specific manner. Forexample, in FIG. 1, the first group of cylinders may include the fourcylinders of the first bank 15 a while the second group of cylinders mayinclude the four cylinders of the second bank 15 b. In an alternateexample, instead of one or more cylinders from each bank beingdeactivated together, two cylinders from each bank of the V8 engine maybe selectively deactivated together.

Engine 10 may operate on a plurality of substances, which may bedelivered via fuel system 8. Engine 10 may be controlled at leastpartially by a control system including controller 12. Controller 12 mayreceive various signals from sensors 4 coupled to engine 10, and sendcontrol signals to various actuators 22 coupled to the engine and/orvehicle.

Fuel system 8 may be further coupled to a fuel vapor recovery system(not shown) including one or more canisters for storing refueling anddiurnal fuel vapors. During selected conditions, one or more valves ofthe fuel vapor recovery system may be adjusted to purge the stored fuelvapors to the engine intake manifold to improve fuel economy and reduceexhaust emissions. In one example, the purge vapors may be directed nearthe intake valve of specific cylinders. For example, during a VDE modeof operation, purge vapors may be directed only to the cylinders thatare firing. This may be achieved in engines configured with distinctintake manifolds for distinct groups of cylinders. Alternatively, one ormore vapor management valves may be controlled to determine whichcylinder gets the purge vapors.

Controller 12 may receive an indication of cylinder knock orpre-ignition from one or more knock sensors 82 distributed along theengine block. When included, the plurality of knock sensors may bedistributed symmetrically or asymmetrically along the engine block. Assuch, the one or more knock sensors 82 may be accelerometers, orionization sensors. Further details of the engine 10 and an examplecylinder are described with regard to FIGS. 2A-C and 3.

FIG. 2A is a schematic depiction of a first embodiment 200 of an exampleturbocharged variable displacement engine system 100 includingmulti-cylinder internal combustion engine 10 (of FIG. 1) and twinturbochargers 120 and 130. Engine system 100 can receive intake air viaintake passage 140. Intake passage 140 may include an air filter 156 andan air intake system (AIS) throttle valve 230. AIS throttle 230 may bepositioned in the engine intake upstream of the first and secondcompressors 122, 132, and upstream of a split point from where the firstand second parallel intake passages 142 and 144 diverge. The position ofAIS throttle 230 may also be adjusted to generate vacuum to help withEGR flow, as described herein.

Engine system 100 may be a split-engine system wherein intake passage140 is branched downstream of AIS throttle valve 230, at a branch point,into first and second parallel intake passages 142 and 144, eachincluding a turbocharger compressor. Specifically, at least a portion ofintake air is directed to compressor 122 of turbocharger 120 via firstparallel intake passage 142 and at least another portion of the intakeair is directed to compressor 132 of turbocharger 130 via secondparallel intake passage 144.

The first portion of the total intake air that is compressed bycompressor 122 may be supplied to intake manifold 160 via first parallelbranched intake passage 146. Herein, intake passages 142 and 146 mayform a first branch of the engine's air intake system. Similarly, asecond portion of the total intake air may be compressed via compressor132 where it may be supplied to intake manifold 160 via second parallelbranched intake passage 148. Herein, intake passages 144 and 148 mayform a second parallel branch of the engine's air intake system. Asshown in FIG. 2A, intake air from intake passages 146 and 148 can berecombined, via a common intake passage 149 before reaching intakemanifold 160. From there, the intake air may be provided to the engine.Intake passage 149 may include an air cooler 154 for cooling boosted airreceived from the turbocharger compressors. Intake manifold 160 mayinclude an intake manifold pressure sensor 182 for estimating a manifoldpressure (MAP) and/or an intake manifold temperature sensor 183 forestimating a manifold air temperature (MCT), each communicating withcontroller 12.

Air intake throttle 158 may be positioned in the engine intakedownstream of the first and second compressors, and downstream of amerge point where first and second parallel branched intake passages 146and 148 merge to form common intake passage 149. When one of thecylinder groups is deactivated (that is, when the engine is operating ina VDE mode), the position of air intake throttle 158 may be adjusted bycontroller 12 to control an amount of air entering the engine, therebyenabling a desired torque to be provided. For example, throttle 158 maybe opened to allow the active cylinder group to generate the same torqueas was previously being generated by the two cylinder groups. During thetransition to the deactivation state, throttle 158 may be opened toallow the intake MAP to increase. At the same time, spark may beretarded (e.g., by a first amount) to maintain a constant torque on allthe cylinders, thereby reducing cylinder torque disturbances. When thereis sufficient MAP, one or more fuel injectors of the selected group ofcylinders may be turned off, while spark timing is restored. In the sameway, during a transition out of the deactivated state (that is, duringreactivation), while the disabled fuel injectors are being turned on,throttle 158 may be closed to allow the intake MAP to decrease. At thesame time, spark may be retarded (e.g., by a second, different amount)to maintain a constant torque on all the cylinders, thereby reducingcylinder torque disturbances. When sufficient MAP is restored, sparktiming may be restored

Engine 10 may include a plurality of cylinders 14. In the depictedexample, engine 10 is shown with six cylinders arranged in aV-configuration. Specifically, the six cylinders are arranged on twobanks (or groups of cylinders) 15 a and 15 b, with each bank includingthree cylinders. In another example, such as elaborated with referenceto FIG. 1, the engine may have four cylinders on each bank. Eachcylinder 14 may be configured with a fuel injector 166. In the depictedexample, fuel injector 166 is a direct in-cylinder injector. However, inother examples, fuel injector 166 can be configured as a port fuelinjector. Further details of a single cylinder 14 are described below inFIG. 3.

Intake air supplied to cylinders 14 via common intake passage 149 may beused for fuel combustion. Products of combustion may then be exhaustedvia bank-specific parallel exhaust passages (as shown) or via a commonexhaust passage. In the depicted example, exhaust from the first groupof cylinders 15 a is directed to first parallel exhaust passage 17 whileexhaust from the second group of cylinders 15 b is directed to secondparallel exhaust passage 19. Each of the first and second parallelexhaust passages 17 and 19 may further include a turbocharger turbine.Specifically, exhaust gas is directed through exhaust passage 17 todrive exhaust turbine 124 of turbocharger 120, which in turn can providemechanical work to compressor 122 via shaft 126 in order to providecompression to the intake air. Alternatively, some or all of the exhaustgas in exhaust passage 17 can bypass turbine 124 via turbine bypasspassage 123 as controlled by wastegate 128. Similarly, exhaust isdirected through exhaust passage 19 to drive exhaust turbine 134 ofturbocharger 130, which in turn can provide mechanical work tocompressor 132 via shaft 136 in order to provide compression to intakeair flowing through the second branch of the engine's intake system.Alternatively, some or all of the exhaust gas in exhaust passage 19 canbypass turbine 134 via turbine bypass passage 133 as controlled bywastegate 138.

An anti-surge valve 152 may be provided to selectively bypass thecompressor stages of turbochargers 120 and 130 via bypass passage 150.As one example, anti-surge valve 152 can open to enable flow throughbypass passage 150 when the intake air pressure upstream of thecompressors attains a threshold value.

In some examples, exhaust turbines 124 and 134 may be configured asvariable geometry turbines, wherein controller 12 may adjust theposition of the turbine impeller blades (or vanes) to vary the level ofenergy that is obtained from the exhaust gas flow and imparted to theirrespective compressor. Alternatively, exhaust turbines 124 and 134 maybe configured as variable nozzle turbines, wherein controller 12 mayadjust the position of the turbine nozzle to vary the level of energythat is obtained from the exhaust gas flow, and imparted to theirrespective compressors. For example, the control system can beconfigured to independently vary the vane or nozzle position of theexhaust gas turbines 124 and 134 via respective actuators.

Exhaust gases in first parallel exhaust passage 17 may be directed tothe atmosphere via branched parallel exhaust passage 170 while exhaustgases in second parallel exhaust passage 19 may be directed to theatmosphere via branched parallel exhaust passage 180. Exhaust passage170 may include an emission control device, such as a first catalyst 302coupled downstream of the first group of cylinders, while exhaustpassage 180 may include an emission control device, such as a secondcatalyst 304 coupled downstream of the second group of cylinders.Various exhaust gas sensors may be coupled to the emission controldevices, as further elaborated in FIG. 3. These may include, forexample, a first temperature sensor (not shown) for estimating atemperature of the first catalyst 302 and a second temperature sensor(not shown) for estimating a temperature of the second catalyst 304.Other sensors may include, for example, a first set of exhaust gasoxygen sensors (not shown) coupled across the first catalyst 302 toestimate a regeneration state of the first catalyst (e.g., based on anair-to-fuel ratio differential across the first catalyst), and a secondset of exhaust gas oxygen sensors (not shown) coupled across the secondcatalyst 304 to estimate a regeneration state of the second catalyst(e.g., based on an air-to-fuel ratio differential across the secondcatalyst). First catalyst 302 may be coupled to first group of cylinders15 a, downstream of turbine 124, via a first shut-off valve 306.Likewise, second catalyst 304 may be coupled to second group ofcylinders 15 b, downstream of turbine 134, via a second shut-off valve308. As elaborated with reference to FIG. 6, by adjusting the positionof the first and/or second shut-off valve based on which cylinder groupis selected for deactivation, fall in catalyst temperature and floodingof the catalyst with oxygen can be reduced, thereby reducing an amountof fuel needed for catalyst regeneration.

While the depicted embodiment shows each exhaust passage includingrespective shut-off valves coupled upstream of respective exhaustcatalysts, in an alternate embodiment (not shown), the exhaust passagesmay be coupled via valves and passages that allow at least a portion ofexhaust gas from the enabled group of cylinders to be directed throughthe catalyst of the disabled group of cylinders. For example, an exhaustbranch passage may couple the first parallel exhaust passage, downstreamof the first catalyst, to the second parallel exhaust passage, upstreamof the second catalyst. When the second cylinder group is deactivated, avalve in the exhaust branch passage may divert at least some exhaust gasfrom the first exhaust passage through the second catalyst, warming thesecond catalyst and reducing the fuel penalty of regeneration.

In still another embodiment (not shown), each exhaust passage mayfurther include a bypass passage for directing air or exhaust to theatmosphere around (but not through) the respective catalysts. Herein,the shut-off valve coupled upstream of the catalyst may be configured asa diverter valve configured to divert pure air from the disabledcylinder group into the bypass passage, and around the catalyst, inresponse to deactivation of the cylinder group.

In yet another embodiment, the engine may be configured with valves andpassages for recirculating the pure air from the disabled cylinder groupback to the intake manifold, without directing the air through thecatalyst. For example, a diverter passage may be configured to directexhaust gas from upstream of the catalyst into the intake manifold.Herein, in addition to keeping the catalyst warm and reducing oxygensaturation of the catalyst coupled to the disabled cylinder group, theexhaust pressure of the disabled cylinder group may be matched to theintake pressure, reducing pumping losses and improving engineefficiency.

While the depicted embodiment shows parallel exhaust passages directingexhaust to respective emission control devices, in alternateembodiments, the catalysts may be configured as split catalyst systemswherein airflow from different cylinders (or different cylinder groups)are directed to different sections of a catalyst brick. Specifically,the split catalyst system may allow hot exhaust from enabled cylindersto flow through one section of the catalyst brick while cooler air fromthe disabled cylinders flows through a different section of the catalystbrick. Heat from the hot exhaust of the enabled cylinders is transferredto the catalyst brick while cooler air from the disabled cylinders coolsthe catalyst. In this way, heat can be transferred from the hot side ofthe catalyst to the cold side, keeping the entire catalyst warm. In oneembodiment, heat transfer may be improved by using a metallic substratefor the catalyst.

Engine 10 may further include one or more exhaust gas recirculation(EGR) loops for recirculating at least a portion of exhaust gas fromfirst and second parallel exhaust passages 17 and 19 and/or first andsecond parallel branched exhaust passages 170 and 180, to first andsecond parallel intake passages 142 and 144, and/or parallel branchedintake passages 146 and 148. These may include high-pressure EGR loopsfor proving high-pressure EGR (HP-EGR) and low-pressure EGR-loops forproviding low-pressure EGR (LP-EGR). In one example, HP-EGR may beprovided in the absence of boost provided by turbochargers 120, 130,while LP-EGR may be provided in the presence of turbocharger boostand/or when exhaust gas temperature is above a threshold. In still otherexamples, both HP-EGR and LP-EGR may be provided simultaneously.

In the depicted example, engine 10 may include a first low-pressure EGRloop 202 for recirculating at least some exhaust gas to the first groupof cylinders 15 a, specifically, from the first branched parallelexhaust passage 170, downstream of the turbine 124, to the firstparallel intake passage 142, upstream of the compressor 122. Likewise,the engine may include a second low-pressure EGR loop 212 forrecirculating at least some exhaust gas to the second group of cylinders15 b, specifically, from the second branched parallel exhaust passage180, downstream of the turbine 134, to the second parallel intakepassage 144, upstream of the compressor 132. First and second LP-EGRloops 202 and 212 may include respective LP-EGR valves 204 and 214 forcontrolling an amount of exhaust gas recirculated (that is, EGR flow)through the loops. The first and second EGR loops may also includerespective charge air coolers 206 and 216 for lowering a temperature ofexhaust gas flowing through the respective EGR loops beforerecirculation into the engine intake. Under certain conditions, thecharge air coolers 206, 216 may also be used to heat the exhaust gasflowing through LP-EGR loops 202, 212 before the exhaust gas enters thecompressor to avoid water droplets impinging on the compressors.

Engine 10 may further include a first high-pressure EGR loop 208 forrecirculating at least some exhaust gas to the first group of cylinders15 a, specifically, from the first parallel exhaust passage 17, upstreamof the turbine 124, to the first branched parallel intake passage 146,downstream of the compressor 122. Likewise, the engine may include asecond high-pressure EGR loop 218 for recirculating at least someexhaust gas to the second group of cylinders 15 b, specifically, fromthe second parallel exhaust passage 180, upstream of the turbine 134, tothe second branched parallel intake passage 148, downstream of thecompressor 132. EGR flow through HP-EGR loops 208 and 218 may becontrolled via respective HP-EGR valves 210 and 220. The EGR loops mayinclude various sensors for estimating the EGR. These may include, forexample, air-to-fuel ratio sensors, temperature sensors, pressuresensors, and humidity sensors.

During a VDE mode of engine operation, exhaust energy generated in theexhaust of the disabled cylinder group is not sufficient to drive thecorresponding turbine. In some cases, boost pressure generated by thecompressor coupled to the enabled cylinder group may backflow to thedisabled cylinder group, overwhelming the weakly operating turbine. Toreduce boost losses resulting from such backflow, one or more backflowvalves or throttles may be included. In one example, as shown in FIG.2A, a first backflow valve 310 may be included in first parallelbranched intake passage 146, downstream of first compressor 122.Additionally, or optionally, a second backflow valve 312 may be includedin second parallel branched intake passage 148, downstream of secondcompressor 132. Backflow valves 310 and 312 may be passive valves, oractive valves. In one example, backflow valves 310 and 312 may be flapvalves. As elaborated with reference to FIG. 7, upon selecting a groupof cylinders for deactivation, an engine controller may close thebackflow valve coupled to the disabled group of cylinders. At the sametime, the controller may adjust a position (or opening) of the backflowvalve coupled to the enabled group of cylinders while operating thecorresponding compressor, to provide a desired amount of boost to theengine. The position of downstream throttle 158 and/or upstream throttle230 may be adjusted concurrently, as elaborated in FIG. 7, to assist inproviding the desired boost, with reduced torque disturbances, inparticular as the selected cylinder group is deactivated or reactivated.

FIG. 2B shows an alternate embodiment 300 of engine system 100 includingthrottles for reducing backflow in place of the backflow valves of FIG.2A. Specifically, embodiment 300 depicts first parallel branched intakepassage 146 including a first backflow throttle valve 320 coupleddownstream of the first turbocharger compressor 122, and second parallelbranched intake passage 148 including a second backflow throttle valve322 coupled downstream of the second turbocharger compressor 132. In thedepicted configuration, in lieu of adjusting throttle 158 in the intakemanifold, the position of the two throttles installed upstream of themerge point of the parallel branched intake passages may be adjusted toprovide an amount of boost while reducing backflow issues (FIG. 7).Thus, in the depicted embodiment, throttle 158 may not be included.

FIG. 2C shows yet another embodiment 350 of engine system 100 alsoincluding throttles for reducing backflow in place of the backflowvalves of FIG. 2A. In the depicted embodiment, the throttles arepositioned upstream of the respective compressors. Specifically,embodiment 350 depicts first parallel intake passage 142 including thefirst backflow throttle valve 320 coupled upstream of the firstturbocharger compressor 122, and second parallel intake passage 144including second backflow throttle valve 322 coupled upstream of thesecond turbocharger compressor 132. In the depicted configuration, inlieu of adjusting AIS throttle valve 230, the two throttles installedupstream of the branch point of the parallel intake passages areadjusted to provide an amount of boost while reducing backflow issues.Thus, in the depicted embodiment, throttle 230 may not be included. Aselaborated in FIG. 7, the throttles may be further adjusted to generatea vacuum in the respectively coupled EGR loops and adjust an amount ofEGR directed there-through. By adjusting the various valves andthrottles based on the selective deactivation of a group of cylinders,VDE transitions may be improved, while also improving boost generationduring a VDE mode of operation.

Returning to FIG. 2A, the position of intake and exhaust valves of eachcylinder 14 may be regulated via hydraulically actuated lifters coupledto valve pushrods, or via a cam profile switching mechanism in which camlobes are used. For example, the intake valves of each cylinder 14, oreach cylinder group (15 a, 15 b), may be controlled by cam actuationusing a cam actuation system. Specifically, the intake valve camactuation system 25 may include one or more cams and may utilizevariable cam timing or lift for intake and/or exhaust valves. Inalternative embodiments, the intake valves may be controlled by electricvalve actuation. Similarly, the exhaust valves may be controlled by camactuation systems or electric valve actuation. Valve actuation iselaborated herein with reference to FIG. 3.

Engine system 100 may be controlled at least partially by a controlsystem 15 including controller 12 and by input from a vehicle operatorvia an input device (not shown). Control system 15 is shown receivinginformation from a plurality of sensors 80 (various examples of whichare described herein) and sending control signals to a plurality ofactuators 81. As one example, sensors 80 may include temperature sensorsand air-to-fuel ratio sensors coupled to the exhaust catalysts, MAPsensor 182, and MAT sensor 183, etc. In some examples, common intakepassage 149 may include a throttle inlet pressure (TIP) sensor forestimating a throttle inlet pressure (TIP) and/or a throttle inlettemperature sensor for estimating a throttle air temperature (TCT). Inother examples, one or more of the EGR passages may include pressure,temperature, and air-to-fuel ratio sensors, for determining EGR flowcharacteristics. Additional system sensors and actuators are elaboratedbelow with reference to FIG. 3. As another example, actuators 81 mayinclude fuel injector 166, HP-EGR valves 210 and 220, LP-EGR valves 204and 214, throttle valves 158 and 230, shut-off valves 306 and 308,back-flow valves (or throttles) 310 and 312, and wastegates 128, 138.Other actuators, such as a variety of additional valves and throttles,may be coupled to various locations in engine system 100. Controller 12may receive input data from the various sensors, process the input data,and trigger the actuators in response to the processed input data basedon instruction or code programmed therein corresponding to one or moreroutines. Example control routines are described herein with regard toFIGS. 4-8.

FIG. 3 depicts an example embodiment 300 of a cylinder or combustionchamber of internal combustion engine 10 (of FIGS. 1 and 2A-C). Engine10 may receive control parameters from controller 12 and input fromvehicle operator 190 via an input device 192, such as an acceleratorpedal and a pedal position sensor 194 for generating a proportionalpedal position signal PP. Cylinder (herein also “combustion chamber”) 14of engine 10 may include combustion chamber walls 236 with piston 238positioned therein. Piston 238 may be coupled to crankshaft 240 so thatreciprocating motion of the piston is translated into rotational motionof the crankshaft. Crankshaft 240 may be coupled to at least one drivewheel of the passenger vehicle via a transmission system. Further, astarter motor may be coupled to crankshaft 240 via a flywheel to enablea starting operation of engine 10.

Cylinder 14 can receive intake air via a series of intake air passages242, 244, and 246. Intake air passage 246 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger 280. For example, FIG. 3 shows engine 10 configured witha turbocharger including a compressor 282 arranged between intakepassages 242 and 244, and an exhaust turbine 284 arranged along exhaustpassage 248. Compressor 282 may be at least partially powered by exhaustturbine 284 via a shaft 286 where the boosting device is configured as aturbocharger. A throttle valve 158 including a throttle plate 164 may beprovided along an intake passage of the engine for varying the flow rateand/or pressure of intake air provided to the engine cylinders. Forexample, throttle valve 158 may be disposed downstream of compressor 282as shown in FIGS. 2A and 2C, or alternatively may be provided upstreamof a compressor. In some embodiments, such as shown in FIG. 2B, throttlevalve 158 may be omitted wherein the intake passage may include one ormore backflow throttle valves for varying the flow rate and/or pressureof the intake air.

Exhaust passage 248 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 228 is showncoupled to exhaust passage 248 upstream of emission control device 278.Sensor 228 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 278 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 248. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 228. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 250 and at least one exhaust poppet valve256 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder. Operation of intake valve 250 andexhaust valve 256 may be controlled by cam actuation via respective camactuation systems 251 and 253. Cam actuation systems 251 and 253 mayeach include one or more cams and may utilize one or more of cam profileswitching (CPS), variable cam timing (VCT), variable valve timing (VVT)and/or variable valve lift (VVL) systems that may be operated bycontroller 12 to vary valve operation. In alternative embodiments, theintake and/or exhaust valve may be controlled by electric valveactuation. In one example, cylinder 14 may include an intake valvecontrolled via cam actuation including VCT systems and an exhaust valvecontrolled via electric valve actuation. In one example, during a VDEmode of operation, when one or more fuel injectors are disabled, thecamshaft timing for the enabled cylinders may be adjusted based on thedesired torque while the camshaft timing for the disabled cylinders isadjusted to reduce pumping losses. Additionally, or optionally, acamshaft timing may be selected for the disabled cylinders that furtherreduces air flow through the disabled cylinder group, thereby loweringthe airflow through the downstream catalyst. Herein, the camshaft timingadjustment may advantageously slow a cooling off of the catalyst.

In some embodiments, each cylinder of engine 10 may include a spark plug292 for initiating combustion. Ignition system 290 can provide anignition spark to combustion chamber 14 via spark plug 292 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 292 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel, as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 14 for injecting fueldirectly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides what is known as direct injection (hereafteralso referred to as “DI”) of fuel into combustion cylinder 14.Alternatively, the injector may be located overhead and near the intakevalve to improve mixing. Fuel may be delivered to fuel injector 166 froma high pressure fuel system 8 including fuel tanks, fuel pumps, and afuel rail. Alternatively, fuel may be delivered by a single stage fuelpump at lower pressure, in which case the timing of the direct fuelinjection may be more limited during the compression stroke than if ahigh pressure fuel system is used. Further, while not shown, the fueltanks may have a pressure transducer providing a signal to controller12. It will be appreciated that, in an alternate embodiment, injector166 may be a port injector providing fuel into the intake port upstreamof cylinder 14.

Controller 12 is shown in FIG. 3 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory (ROM) chip 110 in this particular example, random access memory(RAM) 112, keep alive memory (KAM) 114, and a data bus. Storage mediumread-only memory 110 can be programmed with computer readable datarepresenting instructions executable by processor 102 for performing themethods described below as well as other variants that are anticipatedbut not specifically listed. Controller 12 may receive various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 231; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 260 (or other type)coupled to crankshaft 240; throttle position (TP) from a throttleposition sensor; and absolute manifold air pressure signal (MAP) fromsensor 182. Engine speed signal, RPM, may be generated by controller 12from signal PIP. Further, crankshaft position, as well as crankshaftacceleration, and crankshaft oscillations may also be identified basedon the signal PIP. Manifold air pressure signal MAP from manifoldpressure sensor 182 may be used to provide an indication of vacuum, orpressure, in the intake manifold. Further, as noted herein, manifoldpressure may be estimated based on other operating parameters, such asbased on MAF and RPM, for example.

As described above, FIG. 3 shows only one cylinder of a multi-cylinderengine, and as such, each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector, spark plug, etc.

The engine controller may periodically perform various on-boarddiagnostic tests to verify the functionality of the various valves andsensors of FIGS. 1-3. In one example, such as when the engine is in thenon-VDE mode, the various tests may be performed only duringdeceleration fuel shut-off. In another example, when the engine is inthe VDE mode, diagnostic routines may be executed on the disabled groupof cylinders. In one example, based on an estimated amount of time thatthe engine is expected to operate in the VDE mode, one or morediagnostic tests may be selected (e.g., oxygen sensor control).

FIG. 4 depicts an example routine 400 that may be executed by an enginecontroller to determine a mode of operation of the VDE engine system.Specifically, based on engine operating conditions, it may be determinedwhether the engine is to be run with all cylinders firing (that is,non-VDE mode) or with one or more cylinders deactivated (that is, VDEmode). If the engine is to be operated in the VDE mode, a group ofcylinders and a number of cylinders may be selected for the deactivation(FIG. 5). Based on the selection, the position of various engine systemvalves (FIG. 6) and throttles (FIG. 7) may be adjusted to maintain theoperating conditions (e.g., temperature) of a catalyst coupled to thedisabled group of cylinders during the deactivation, as well as tocompensate for torque and/or EGR transients during the transitioning into and/or out of the VDE mode of operation. In this way, during a VDEmode of operation, an emission control device can be maintained readyfor operation in the non-VDE mode, and the engine can be smoothlytransitioned between VDE and non-VDE modes.

At 402, the routine includes measuring and/or estimating engineoperating conditions. The conditions assessed may include barometricpressure, a driver-demanded torque (for example, from a pedal-positionsensor), manifold pressure (MAP), manifold air flow (MAF), enginetemperature, air temperature, knock limits, etc.

At 404, based on the estimated operating conditions, the routinedetermines an engine mode of operation (e.g., VDE or non-VDE). In oneexample, where the engine is a V8 engine, during the VDE mode, theengine may be operated with only one group of cylinders activated (thatis, in a V4 mode) while during the non-VDE mode, the engine may beoperated with both groups of cylinders activated (that is, in a V8mode).

At 406, it may be confirmed whether the engine is to be operated in theVDE (e.g., V4) mode. If not, the routine may end. If yes, then at 408,the routine includes selecting a group of cylinders for deactivation. Inone example, such as with reference to the engine system of FIGS. 1-3,the engine may include a first and a second group of cylinders, and anengine controller may select either the first or the second group ofcylinders for deactivation. As elaborated with reference to FIG. 5, theselection may be based on a regeneration state of a first catalystcoupled to the first group of cylinders relative to a second catalystcoupled to the second group of cylinders. Additionally, the selectionmay be further based on a temperature of the first catalyst relative tothe second catalyst, as well as a deactivation order. Following theselection, the selected group of cylinders may be deactivated. Herein,the deactivation may include turning off fuel injectors while continuingto open or close intake and exhaust valves so as to pump air through theselected group of cylinders.

At 410, and as further elaborated at FIG. 6, based on the deactivation,the position of one or more engine system valves may be adjusted. Theone or more valves that are adjusted may include EGR valves (e.g.,valves 204, 214, 210 and 220 of FIG. 2A) as well as one or more shut-offvalves (e.g., valves 306 and 308 of FIG. 2A). At 412, and as furtherelaborated at FIG. 7, based on the deactivation, the position of one ormore engine system throttles may be adjusted. The one or more throttlesthat are adjusted may include intake throttles upstream of theturbocharger compressor (e.g., throttles 320 and 322 of FIG. 2C) as wellas one or more throttles downstream of the turbocharger compressor(e.g., throttles 320 and 322 of FIG. 2B).

At 414, it may be determined whether any abnormal cylinder combustionevent related to knock and/pre pre-ignition has been detected. Aselaborated at FIG. 8, based on the deactivation, in particular thenumber of deactivated cylinders, each of a threshold and a window forknock detection and pre-ignition detection may be adjusted. As such, thebackground noise level may vary based on the number of firing cylinders.Thus, by adjusting the windows and thresholds based on the deactivation,abnormal combustion events with small noise levels, occurring duringnon-VDE modes of engine operation, as well as normal combustion eventswith large noise levels, occurring during VDE modes of engine operation,may be better distinguished.

If knock and/or pre-ignition is detected, then at 416, an appropriatemitigating action may be taken. For example, in response to cylinderknock, a spark timing adjustment (e.g., spark retard) may be performed.In comparison, in response to cylinder pre-ignition, a cylinder fuelinjection adjustment (e.g., cylinder enrichment or cylinder enleanmentfor a duration) may be performed.

In this way, identification and distinction of abnormal combustionevents may be improved, and better mitigated. By reducing the occurrenceof cylinder knock and pre-ignition, engine degradation can be reducedwhile improving fuel economy.

Now turning to FIG. 5, an example routine 500 is shown for selecting agroup of cylinders for deactivation based at least on a regenerationstate of engine catalysts. The selection may be further based on atemperature of the catalysts as well as a deactivation order. Byselecting a group of cylinders based on the regeneration state andtemperature of the catalysts, fuel wastage related to completion ofregeneration may be reduced. By selecting a group of cylinders based onthe deactivation order, disablement of groups of cylinders may bealternated to balance engine and catalyst wear between the cylinders.

At 502, the routine includes determining the deactivation order of afirst and a second group of cylinders. In one example, the deactivationorder may be stored in the memory of a controller and updated afterevery deactivation (or VDE) cycle. The deactivation order may indicatewhich group of cylinders was deactivated in the preceding cycle(s), aswell as how many cylinders of the selected cylinder group (and theidentity of the cylinders) were deactivated during each precedingdeactivation cycle.

At 504, the routine includes determining the regeneration state of thefirst catalyst coupled to the first group of cylinders and the secondcatalyst coupled to the second group of cylinders. In one example, theregeneration state of the first catalyst is estimated based on anair-to-fuel ratio differential across the first catalyst. Theair-to-fuel ratio differential may be estimated by a first set ofexhaust gas sensors (e.g., oxygen sensors) coupled across the firstcatalyst (e.g., one sensor upstream of, and another sensor downstream ofthe first catalyst). Likewise, the regeneration state of the secondcatalyst may be estimated based on an air-to-fuel ratio differentialacross the second catalyst, as determined by a second set of exhaust gassensors (e.g., oxygen sensors) coupled across the second catalyst (e.g.,one sensor upstream of, and another sensor downstream of the secondcatalyst). In an alternate example, the regeneration state of eachcatalyst may be based on an oxygen content of the catalyst, as estimatedby an oxygen sensor coupled to the respective catalysts.

At 506, the regeneration state of the first catalyst may be compared tothe regeneration state of the second catalyst. Specifically, it may bedetermined if the first catalyst is less regenerated than the secondcatalyst, for example, by at least a threshold amount (e.g., 20%). Inone example, the first catalyst may be partially regenerated while thesecond catalyst is fully regenerated. In one embodiment of routine 500,wherein the selection is based on the regeneration state of thecatalysts only, the routine may include selecting the first group ofcylinders to be deactivated (at 514) when the first catalyst is lessregenerated than the second catalyst (at 506).

If the first catalyst is not less regenerated than the second catalyst(at 506), then at 508, it may be confirmed if the second catalyst isless regenerated than the first catalyst, for example, by at least athreshold amount (e.g., 20%). In one example, the second catalyst may bepartially regenerated while the first catalyst is fully regenerated. Inthe embodiment of routine 500 wherein the selection is based on theregeneration state of the catalysts only, the routine may includeselecting the second group of cylinders to be deactivated (at 516) whenthe second catalyst is less regenerated than the first catalyst (at508).

If the second catalyst is not less regenerated than the first catalyst(at 508), then at 510, it may be confirmed if the first and secondcatalysts are both sufficiently regenerated. For example, it may bedetermined if the first and second catalysts are both regenerated by atleast a threshold amount, and/or a difference between their regenerationstates is less than a threshold amount. In one example, both the firstand second catalyst may be fully regenerated. When the first and secondcatalysts are both sufficiently regenerated, at 520, the routineincludes selecting a group of cylinders based on the deactivation order.That is, selecting the group of cylinders that was not previouslydeactivated (that is, was not deactivated on the immediately previousdeactivation cycle). By alternating the group of cylinders that isselected over consecutive deactivation cycles, catalyst and engine wearmay be balanced. For example, if the first group of cylinders wasselectively deactivated on the immediately preceding deactivation cycle,the second group of cylinders may be selectively deactivated on afollowing (e.g., the next) deactivation cycle. Likewise, if the secondgroup of cylinders was selectively deactivated on a first deactivationcycle, the first group of cylinders may be selectively deactivated onthe immediately following deactivation cycle.

In this way, the regeneration state of the catalyst may be used tooverride a default deactivation order based selection of cylindersduring a VDE mode of operation. By selecting the group of cylinders withthe partially regenerated catalyst for deactivation, fuel that may haveotherwise been used to complete regeneration of that catalyst may besaved. Thus, fuel economy may be improved.

In another embodiment, the selection may be further based on atemperature of the first catalyst relative to the second catalyst. Forexample, to maintain an emission control device at a desired temperature(or within a desired temperature range) during the VDE mode of engineoperation, the selection may be further adjusted based on theregeneration state and the temperature of the catalyst, giving priorityto the temperature. Herein, returning to 506, if the first catalyst isless regenerated than the second catalyst, it may be further confirmedif the temperature of the first catalyst is lower than the temperatureof the second catalyst at 512. Alternatively, it may be determined ifthe first catalyst temperature is lower than a threshold temperature,such as a temperature below which the efficiency of the first catalystdrops below a desired level. The temperature of the first and secondcatalysts may be determined by first and second temperature sensorscoupled to the respective catalysts. If the first catalyst has a lowertemperature and a lower regeneration state, then the first cylindergroup may be selected at 514. However, when the temperature of thesecond catalyst is lower than the temperature of the first catalyst,even if the first catalyst is less regenerated than the second catalyst,the second group of cylinders may be selected for deactivation.Likewise, returning to 508, if the second catalyst is less regeneratedthan the first catalyst, it may be further confirmed if the temperatureof the second catalyst is lower than the temperature of the firstcatalyst at 518. Alternatively, it may be determined if the secondcatalyst temperature is lower than a threshold temperature, such as atemperature below which the efficiency of the second catalyst dropsbelow a desired level. If the second catalyst has a lower temperatureand a lower regeneration state, then the second cylinder group may beselected at 516. However, when the temperature of the first catalyst islower than the temperature of the second catalyst, even if the secondcatalyst is less regenerated than the first catalyst, the first group ofcylinders may be selected for deactivation at 514.

In this way, by selecting a group of cylinders based on a regenerationstate of the catalyst during some conditions and further based on acatalyst temperature during other conditions, exhaust conditions of anemission control device coupled to a deactivated cylinder group may bemaintained and fuel wastage may be reduced.

In one example, the regeneration state and/or temperature based overrideof the default (deactivation order based) cylinder group selection maybe enabled for a threshold number of cycles, after which the selectionmay resume the deactivation order basis to balance catalyst usage. Inother words, the first group of cylinders may be selected fordeactivation for a threshold number of consecutive deactivation cycleswhere the first catalyst is less regenerated than the second catalyst.After the threshold number of consecutive deactivation cycles, thesecond group of cylinders may be selected even if the first catalyst is(still) less regenerated than the second catalyst. Likewise, the secondgroup of cylinders may be selected for deactivation for a thresholdnumber of consecutive deactivation cycles where the second catalyst isless regenerated than the first catalyst. After the threshold number ofconsecutive deactivation cycles, the first group of cylinders may beselected even if the second catalyst is (still) less regenerated thanthe first catalyst

In one example, an engine controller may selectively deactivate a firstgroup of cylinders on a first deactivation cycle (that is, first VDEcycle). On the next deactivation cycle, during a first mode (or firstcondition), the controller may selectively deactivate a second group ofcylinders while during a second mode (or second condition), thecontroller may selectively deactivate the first group of cylindersagain. Deactivating the first group of cylinders may include selectivelydisabling (one or more) fuel injectors while continuing valve operationof the first group of cylinders Likewise, deactivating the second groupof cylinders may include selectively disabling (one or more) fuelinjectors while continuing valve operation of the second group ofcylinders. The first mode may include each of a first exhaust catalyst,coupled downstream of the first group of cylinders, and a second exhaustcatalyst, coupled downstream of the second group of cylinders, beingsufficiently regenerated and/or a difference between their regenerationstates being less than a threshold. In comparison, the second mode mayinclude the first catalyst being less regenerated than the secondcatalyst by more than a threshold amount. In another example, the firstmode may include a temperature of the second catalyst being lower thanthe first catalyst and the first catalyst being less regenerated thanthe second catalyst, while the second mode includes a temperature of thefirst catalyst being lower than the second catalyst and the secondcatalyst being less regenerated than the first catalyst. In this way,the conditions of an exhaust catalyst coupled to a deactivated cylindergroup may be maintained to keep the catalyst operation ready during aneventual cylinder group reactivation.

It will be further appreciated that if a deceleration fuel shut-off(DFSO) is requested while in the VDE mode of engine operation, theengine controller may directly transition the engine from the VDE modeto the DFSO mode without performing an interim catalyst regeneration tofurther reduce fuel wastage.

During selected conditions, when regeneration of a catalyst is required,the catalyst may be regenerated using a two-stage regeneration process.As such, regeneration may be needed to reduce the oxygen content of thecatalyst so that catalyst sites are available for NOx conversion. Thecatalyst regeneration may include, for example, a first enrichment for afirst duration immediately followed by a second enrichment for a secondduration. The first enrichment may have a higher degree of richness ascompared to the second enrichment, and the first duration may be shorterthan the second duration. The switch point from the very rich enrichmentto the slightly rich enrichment may be based on an estimated totaloxygen storage capacity of the catalyst. As such, if the rich stage istoo short, regeneration times may be prolonged. However, if the richstage is too long, unburned hydrocarbons may breakthrough, degradingexhaust emissions. Since the enrichment that can be tolerated beforeexhaust breakthrough will vary based on operating conditions andcatalyst oxygen storage capacity, the engine control system may use anadaptive storage model to predict a total amount of fuel that can beused for the regeneration, as well as to trigger the switch between theregeneration stages at a switch point where the regeneration is adequateand the risk of breakthrough is low. Since the oxygen storage capacityvaries with catalyst temperature and age, the adaptive model may assumea nominal variation with catalyst temperature and learn the adaptivecorrection during regeneration based on the amount of fuel required. Thelearned values may also be used to diagnose the catalyst duringdiagnostic routines.

Now turning to FIG. 6, an example routine 600 is described for adjustingeach of a first shut-off valve, coupled between a first cylinder groupand a first downstream catalyst, and a second shut-off valve, coupledbetween a second cylinder group and a second downstream catalyst, basedon the selective deactivation of either the first cylinder group or thesecond cylinder group. Specifically, based on the selection, theshut-off valve coupled between the selected cylinder group and thedownstream catalyst may be closed while the shut-off valve coupled tothe enabled cylinder group is adjusted to provide an amount of EGR.Additionally, an EGR valve coupled to the enabled cylinder group may beadjusted. As such, the routine of FIG. 6 may be executed when the engineis operated in the VDE mode. By adjusting the various shut-off valvesand EGR valves, VDE operations and EGR operations may be coordinatedwhile maintaining catalyst conditions.

At 602, it may be confirmed that the engine is operating in the VDEmode. Upon confirmation, at 604, the routine includes determining whichcylinder group has been selected for deactivation. As elaborated in FIG.5, either the first or the second cylinder group may have been selectedfor deactivation based on one or more of the regeneration state of afirst catalyst (coupled to the first cylinder group) relative to asecond catalyst (coupled to the second cylinder group), a temperature ofthe first catalyst relative to the second catalyst, and a deactivationorder of the cylinder groups. When the first group of cylinders isselected, for example, when the first catalyst is only partiallyregenerated, one or more fuel injectors from the first group ofcylinders may be deactivated. When the second group of cylinders isselected, for example, when the second catalyst is only partiallyregenerated, one or more fuel injectors from the second group ofcylinders may be deactivated.

At 606, the routine includes, based on the selection, closing the valvecoupled between the selected cylinder group and a downstream catalyst.For example, when the first cylinder group is selected, the routineincludes closing a first shut-off valve coupled between the firstcylinder group and a first downstream catalyst. As another example, whenthe second cylinder group is selected, the routine includes closing asecond shut-off valve coupled between the second cylinder group and asecond downstream catalyst.

At 608, the routine includes opening an EGR valve in the EGR loop of theenabled cylinder group based on a desired amount of EGR. In one example,the EGR valve of the enabled cylinder group may be fully opened. Inanother example, the opening of the EGR valve may be increased toincrease an amount of EGR provided to the enabled cylinder group. At610, the routine includes, adjusting the shut-off valve coupled to theenabled cylinder group to adjust the amount of exhaust gas that isrecirculated through the EGR loop of the enabled cylinder group.

For example, when the first cylinder group is not selected fordeactivation, the routine includes adjusting the first shut-off valve toadjust a first amount of exhaust gas that is recirculated via a firstEGR loop of the first cylinder group. In one example, the first valvemay be adjusted while a first EGR valve in the first EGR loop is fullyopened. The first EGR loop may be coupled upstream of the first shut-offvalve and may be configured to recirculate at least some exhaust gas toan intake of the first group of cylinders. The first amount of EGR maybe based on engine operating conditions, such as, a number ofdeactivated fuel injectors in the second group of cylinders. As anotherexample, when the second cylinder group is not selected fordeactivation, the routine includes adjusting the second shut-off valveto adjust a second amount of exhaust gas that is recirculated via asecond EGR loop of the second cylinder group. The second valve may beadjusted while a second EGR valve in the second EGR loop is fullyopened. The second EGR loop may be coupled upstream of the secondshut-off valve and may be configured to recirculate at least someexhaust gas to an intake of the second group of cylinders. The secondamount of EGR may be based on engine operating conditions, such as, anumber of deactivated fuel injectors in the first group of cylinders.

Each of the first and second EGR valves, and the first and secondshut-off valves may also be adjusted based on EGR transients and torquedisturbances anticipated during a transition of the engine in to and/orout of the VDE mode. Thus, as the first cylinder group is deactivatedand/or reactivated, each of the second shut-off valve and the second EGRvalve may be adjusted to compensate for disturbances in the secondcylinder group. For example, a first amount of EGR provided to the firstcylinder group may be adjusted based on the whether the second group ofcylinders is being deactivated or reactivated. Likewise, when the secondcylinder group is deactivated and/or reactivated, the first EGR valveand the first shut-off valve may be adjusted to compensate fordisturbances in the first cylinder group. For example, the second amountof EGR provided to the second cylinder group may be adjusted based onthe whether the first group of cylinders are being deactivated orreactivated.

In addition to valve adjustments, one or more other engine operatingparameters, such as an amount of boost, spark timing, etc., may also beadjusted to compensate for EGR and torque transients. Similarly, one ormore air intake throttles and backflow throttle valves may be adjustedin coordination with the EGR valves and/or shut-off valves to compensatefor torque disturbances. In one example, a first air intake throttle maybe coupled to the first group of cylinders while a second intakethrottle is coupled to the second group of cylinders, in a parallel airintake system (as shown in FIGS. 2B-C). The engine controller may beconfigured to adjust the first intake throttle when transitioning theone or more fuel injectors of the second group of cylinders through thedeactivation, and adjust the second intake throttle when transitioningthe one or more fuel injectors of the first group of cylinders throughthe deactivation. The air intake throttles may be adjusted differentlywhen transitioning through the deactivation as compared to whentransitioning through a reactivation of the disabled cylinders.

As an example, during a first condition, when the first catalyst is lessregenerated than the second catalyst, the first cylinder group and notthe second cylinder group may be deactivated. During a second condition,when the second catalyst is less regenerated than the first catalyst,the second cylinder group and not the first cylinder group may bedeactivated. During the first condition, the first shut-off valve may beclosed while the second shut-off valve is adjusted to adjust an amountof EGR provided to the second group of cylinders via a second EGR loopincluding a second EGR valve. In the same way, during the secondcondition, the second shut-off valve may be closed while the firstshut-off valve is adjusted to adjust an amount of EGR provided to thefirst group of cylinders via a first EGR loop including a first EGRvalve. During the first condition, the second EGR valve may be fullyopen while during the second condition, the first EGR valve may be fullyopen.

Further, during the first condition, as the first cylinder isdeactivated (that is, transitioned in to the VDE mode), an opening ofthe second shut-off valve may be increased to increase the amount of EGRprovided to the second cylinder group during the transition. In the sameway, during the second condition, as the second cylinder is deactivated(that is, transitioned in to the VDE mode), an opening of the firstshut-off valve may be increased to increase the amount of EGR providedto the first cylinder group during the transition. In an alternateexample, when the first cylinder is deactivated (that is, transitionedout of the VDE mode), an opening of the second shut-off valve may bedecreased to decrease the amount of EGR provided to the second cylindergroup during the transition, while during the second condition, as thesecond cylinder is deactivated (that is, transitioned out of the VDEmode), an opening of the first shut-off valve may be decreased todecrease the amount of EGR provided to the first cylinder group duringthe transition.

In this way, by adjusting and coordinating the operation of one or morevalves coupled to the groups of cylinders based the selectivedeactivation of the cylinders, VDE and EGR operations may be coordinatedand the engine may be better transitioned between VDE and non-VDE modes.By using the same set of valves to maintain catalyst conditions as wellas to adjust EGR amounts, component reduction benefits may also beachieved.

Now turning to FIG. 7, an example routine 700 is described for adjustingeach of a first backflow throttle, coupled upstream of a firstturbocharger compressor in a first intake passage leading to a firstcylinder group, and a second backflow throttle, coupled upstream of asecond turbocharger compressor in a second, parallel intake passageleading to a second cylinder group (such as backflow throttles 320 and322 of FIG. 2C). Specifically, following selection of a cylinder groupfor deactivation, the first and second throttles may be adjusted basedon the deactivation to reduce back-flow of boosted air through thedisabled group of cylinders. In this way, loss of boosting efficiencydue to back-flow may be reduced. As such, the routine of FIG. 7 may beexecuted when the engine is operated in the VDE mode.

At 702, it may be confirmed that the engine is operating in the VDEmode. Upon confirmation, at 704, the routine includes determining whichcylinder group has been selected for deactivation. As elaborated in FIG.5, a controller may select either the first or the second group ofcylinders for deactivation based at least on a regeneration state of thefirst catalyst coupled downstream of the first group of cylindersrelative to the second catalyst coupled downstream of the second groupof cylinders.

At 706 and 708, the routine includes, based on the selection, adjustingthe first backflow throttle coupled upstream of the first compressor inthe first intake passage of the first group of cylinders, and the secondbackflow throttle coupled upstream of the second compressor in thesecond intake passage of the second group of cylinders. Specifically,when the first group of cylinders is selected for deactivation, theroutine includes closing the first backflow throttle valve and at leastpartially opening the second backflow throttle valve. The opening of thesecond throttle may be based on a number of deactivated cylinders in thefirst cylinder group. The opening of the second throttle may also beadjusted based on a desired boost level, and based on an amount ofexhaust gas to be recirculated to the second group of cylinders. Forexample, as the amount of exhaust gas to be recirculated increases, anopening of the second throttle may be increased.

The opening of the second throttle may be further adjusted based onwhether the first group of cylinders is transitioning in to or out ofthe deactivation. For example, the opening of the second throttle may beincreased as the first group of cylinders transition in to thedeactivation, while the opening of the second throttle may be decreasedas the first group of cylinders transition out of the deactivation.

In the same way, when the second group of cylinders is selected fordeactivation, the routine includes closing the second throttle and atleast partially opening the first based on a number of deactivatedcylinders in the second cylinder group, a desired boost level, and adesired amount of EGR to be recirculated to the first cylinder group. Byclosing the throttle coupled to the cylinder group selected fordeactivation, backflow issues can be reduced, thereby reducing boostlosses. By adjusting the backflow throttle coupled to the enabledcylinder group, the desired boost may be provided while compensating fortorque disturbances.

At 710, the controller may adjust the position of a third throttle (suchan intake throttle 158 of FIG. 2C) positioned downstream of the firstand second compressors based on the deactivation to assist in providingthe desired boost and desired amount of EGR. For example, when the firstcylinder group is selected for deactivation, the controller may at leastpartially opening the third throttle based on the number of deactivatedcylinders in the first cylinder group and the opening of the secondthrottle.

In addition to throttle adjustments, other operating parameters may beadjusted during the transition in to or out of the VDE mode ofoperation. For example, the controller may retard spark ignition timingby a first amount as the first group of cylinders transitions in to thedeactivation, and retard spark ignition timing by a second, differentamount as the first group of cylinders transition out of thedeactivation.

While the routine of FIG. 7 describes throttle adjustments for backflowthrottle valves coupled upstream of respective compressors, in analternate embodiment, throttle adjustments may be made for backflowthrottle valves coupled downstream of respective compressors (such asbackflow throttles 320 and 322 of FIG. 2B). For example, the controllermay deactivate the first cylinder group (in response to the firstcatalyst being less regenerated than the second catalyst), followingwhich the controller may adjust each of the first backflow throttle(positioned downstream of the first turbocharger compressor in the firstintake passage leading to the first cylinder group) and the secondbackflow throttle valve (positioned downstream of the secondturbocharger compressor in the second intake passage leading to thesecond cylinder group) responsive to the deactivation. As an example,the controller may fully close the first downstream backflow throttlevalve while adjusting an opening of the second downstream backflowthrottle valve based on the number of deactivated cylinders in the firstcylinder group, and further based on a desired amount of boost. Thecontroller may further adjust a third throttle positioned upstream ofthe first and second compressors (such as IAS throttle 230 in FIG. 2B)responsive to the deactivation, and further based on the opening of thesecond backflow throttle valve and the desired amount of boost. Theopening of the second backflow throttle valve may also be adjustedduring the transition into the VDE mode. Thus, as the first cylindergroup is transitioned to a deactivated state, an opening of the secondbackflow throttle valve may be increased, while decreasing an opening ofthe second backflow throttle valve as the first cylinder group isreactivated.

In still another embodiment, wherein the engine system includes backflowvalves in place of backflow throttles (such as backflow valves 310 and312 of FIG. 2A), the controller may adjust the position of the valvesbased on the deactivation. For example, upon deactivating the firstgroup of cylinders, the controller may close (e.g., fully close) a firstbackflow valve coupled downstream of the first compressor in the firstintake passage coupled to the first group of cylinders. By closing thebackflow valve coupled to the disabled group of cylinders, backflow maybe reduced. Concurrently, the controller may adjust a position of asecond backflow valve coupled downstream of the second compressor in thesecond intake passage coupled to the second group of cylinders, as wellas a throttle valve (e.g., throttle 158 of FIG. 2A) coupled downstreamof the first and second valves. By adjusting the second backflow valveand the throttle valve, while operating the second compressor, a desiredamount of boost may be provided to the enabled second group ofcylinders. The controller may further adjust the position of the secondvalve and the throttle valve based on a number of deactivated cylindersin the first group of cylinders, and a first amount of EGR provided tothe first group of cylinders via a first EGR loop. By adjusting thepositions of the valves and throttles based on the EGR amount, EGRtransients generated during VDE transitions may be reduced, improvingengine performance. Also, VDE operations may be coordinated with EGRoperations.

While the routines of FIGS. 6-7 illustrate adjusting various valves andthrottles during the VDE transitions, it will be appreciated that instill further embodiments, the controller may adjust one or moretransmission components during the transition. For example, thecontroller may allow a transmission converter clutch to slip during thetransition into the VDE and/or during the VDE mode of operation. Thisallows some of the undesirable torque pulses to be absorbed by theconverter, reducing NVH issues experienced by the driver.

It will be appreciated that in addition to adjusting the various valvesand throttles during the VDE transitions, the controller may furtheradjust the VDE transitions according to a performance based schedulethat is based on the vehicle operator's driving habits. The drivinghabits may be learned with respect to, for example, a pedal input, and,if available, a user selectable mode switch (e.g., a switch withpositions for varying degrees of performance mode and fuel economymode). In one example, for a driver with a “busy foot” (e.g., a moreaggressive driver), while in VDE mode of operation, an exit from the VDEmode may be made more difficult, to prevent frequent entry/exit eventsthat may decrease the overall fuel economy. As another example, for thedriver with the “busy foot”, while in a non-VDE mode of operation, anentry into the VDE mode may be made more difficult, to prevent frequententry/exit events that may decrease the overall fuel economy. In thisway, by adjusting the transitions based on operator driving habits,further fuel economy benefits can be achieved.

Now turning to FIG. 8, an example routine 800 for detecting anddifferentiating abnormal combustion events in a variable displacementengine system is described. By adjusting knock and pre-ignitiondetection windows and thresholds based on the deactivation, incorrectidentification of regular combustion events as abnormal combustionevents, in particular in a boosted VDE engine during a VDE mode ofoperation, may be reduced.

At 802, the routine includes determining the mode of engine operation(that is, VDE or non-VDE mode). At 804, a number of deactivatedcylinders in the selected group of cylinders may be determined. At 806,the routine includes adjusting a threshold for each of knock detectionand pre-ignition detection based on the number of deactivated cylinders.At 808, the routine includes adjusting a window for each of knockdetection and pre-ignition detection based on the number of deactivatedcylinders. The window for pre-ignition and the window for knock may bothbe crank angle windows.

As such, the engine controller may be configured to detect anddifferentiate abnormal combustion events due to cylinder knocking fromthose indicative of cylinder pre-ignition based on the output (e.g.,signal timing, amplitude, intensity, frequency, etc.) of the one or moreknock sensors 82 (FIG. 1) distributed along the engine block. By usingdistinct thresholds and windows for knock and pre-ignition detection,knocking may be better differentiated from pre-ignition. Since knockingand pre-ignition require distinct mitigating actions, by improving theirdetection and differentiation, their mitigation can also be improved.Specifically, since pre-ignition is mitigated with fuel injectionadjustments while knock is mitigated with spark timing adjustments, byimproving the detection of cylinder knock and/or pre-ignition, fueleconomy may be improved. In one example, a cylinder pre-ignition eventmay be determined based on the output of the one or more knock sensorsestimated in a first, earlier window being larger than a first, higherthreshold, while a cylinder knock event may be determined based on theoutput of the one or more knock sensors estimated in a second, laterwindow being larger than a second, lower threshold.

The adjustments at 806 and 808 may include, for example, increasing thethreshold for each of knock and pre-ignition as the number ofdeactivated cylinders increases. Likewise, the window for each of knockand pre-ignition may be increased, or widened, as the number ofdeactivated cylinders increases. In other words, when a larger number ofcylinders are deactivated, the knock and pre-ignition windows may covera larger crank angle window, while when a smaller number of cylindersare deactivated, the knock and pre-ignition windows may cover a smallercrank angle window. In one example, where the window for knock detectionand the window for pre-ignition detection are partially overlapping, theadjustment may include increasing an amount of overlap between the knockand pre-ignition detection windows as the number of deactivatedcylinders increases.

As such, when more cylinders are deactivated, the average backgroundnoise level of cylinder combustion events may be lower. Since the enginecontrol system uses the average background noise level as a referencefor determining the threshold against which abnormal combustion eventsare identified, the lower average noise level during cylinderdeactivation can artificially increase the number of cylinder combustionevents identified as knocking or pre-ignition events. The subsequentmitigating actions may reduce fuel economy, engine output, and engineefficiency. Thus, by increasing the threshold when fewer cylinders areenabled, a larger difference between the background noise level and theoutput of the knock sensors may be required for a combustion event to beconsidered an abnormal combustion event. In the same way, by using awider detection window, a timing of the abnormal combustion event may bebetter correlated with engine operating conditions, thereby enablingknock to be better differentiated from pre-ignition.

In one example, an engine controller may have different knock detectionand pre-ignition detection windows and thresholds for the differentmodes of operation stored in a look-up table in the controller's memory.These may include a first threshold and first window for knock detectionwhen the engine is in the non-VDE mode (with all cylinders firing), asecond threshold and second window for knock detection when the engineis in the VDE mode with a smaller number of deactivated cylinders, and athird threshold and third window for knock detection when the engine isin the VDE mode with a larger number of deactivated cylinders.Similarly, a set of windows and thresholds for pre-ignition detectionmay be predetermined and stored for the non-VDE mode, and for a VDE modewith different numbers of deactivated cylinders.

In addition to storing data pertaining to knock and pre-ignition, thecontroller may also store data (e.g., acceleration profiles, signalcontent, etc.) pertaining to cylinder misfires. These may be learned ona per-cylinder basis and used to correct for engine speed profilesirregularities, to improve noise signal quality, and improve noisedetection capabilities.

Following the adjustments to the knock and pre-ignition detectionwindows and thresholds, at 810, it may be determined if the output ofthe one or more knock sensors in the knock detection window is greaterthan the knock detection threshold. If yes, then at 812, cylinder knockmay be determined. Accordingly, at 814, a knock mitigating action may beperformed. For example, spark timing may be retarded in the affectedcylinder, or group of cylinders. If no knock is determined at 810, thenat 816, it may be determined if the output of the one or more knocksensors in the pre-ignition detection window is greater than thepre-ignition detection threshold. If yes, then at 818, cylinderpre-ignition may be determined. Accordingly, at 820, a pre-ignitionmitigating action may be performed. For example, the affected cylinder,or group of cylinders, may be enriched, or enleaned, for a duration.Additionally, or optionally, the affected group of cylinders may be loadlimited.

In one example, a boosted variable displacement engine may include afirst knock sensor coupled to the first group of cylinders (or firstbank) and a second knock sensor coupled to the second group of cylinders(or second bank). The engine controller may estimate the output of theknock sensors relative to a first knock threshold in a first knockwindow and relative to a second pre-ignition threshold in a secondpre-ignition window. Cylinder knock and/or pre-ignition may bedetermined based on the estimation. The first and second windows and thefirst and second thresholds may be adjusted based on the group ofcylinders selected for deactivation. As an example, when the first groupof cylinders is selected for deactivation, the first and secondthresholds may be adjusted based on an output of the second knock sensorbut not the first knock sensor. Likewise, when the second group ofcylinders is selected for deactivation, the first and second thresholdsmay be adjusted based on an output of the first knock sensor but not thesecond knock sensor. In other words, only the output of the knock sensorcoupled to the enabled group of cylinders may be used to adjust theknock and pre-ignition thresholds, while the output of the knock sensorcoupled to the disabled group of cylinders may be disregarded. Asanother example, the first and second thresholds may be adjusted basedon an average output of the first and second knock sensor when neithergroup of cylinders are deactivated (that is, based on the output of theknock sensors when the engine is in the non-VDE mode only). In this way,by adjusting the threshold, or reference noise levels, against whichabnormal combustion events are identified based on the deactivation, theerroneous identification of combustion events as cylinder knock orpre-ignition combustion events may be reduced. As such, this allowsengine degradation to be reduced and fuel economy to be improved.

In this way, by adjusting the operation of a variable displacementengine, fuel economy can be improved. By selecting the group ofcylinders that is coupled to the less regenerated catalyst, even if thesame group of cylinders was selected in the immediately previous VDEcycle, fuel wastage due to catalyst regeneration on a disabled cylindergroup can be reduced. In this way, fuel economy may be improved withoutsubstantially affecting engine or catalyst wear.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,1-4, 1-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method of operating an engine,comprising: determining relative regeneration states of a first andsecond catalyst respectively coupled to a first group of cylinders and asecond group of cylinders; and selecting either the first group or thesecond group of cylinders for deactivation based on whether the firstand second catalysts are both regenerated by at least a first threshold,and a difference between their regeneration states is less than a secondthreshold.
 2. The method of claim 1, wherein the regeneration state ofthe first catalyst is estimated based on an air-to-fuel ratiodifferential across the first catalyst, and wherein the regenerationstate of the second catalyst is estimated based on an air-to-fuel ratiodifferential across the second catalyst, wherein the deactivationincludes turning off fuel injectors and continuing to open or closeintake and exhaust valves to pump air through the selected group ofcylinders.
 3. The method of claim 1 wherein the selection overrides adefault deactivation order based selection of cylinders.
 4. The methodof claim 1 wherein the selection includes selecting cylinders based on adeactivation order when both catalysts are regenerated by at least thefirst threshold, and the difference between their regeneration states isless than the second threshold.
 5. The method of claim 1, wherein theselecting includes, selecting the first group of cylinders to bedeactivated when the first catalyst is less regenerated than the secondcatalyst.
 6. The method of claim 5, wherein the selecting furtherincludes, selecting the first group of cylinders to be deactivated whenthe first catalyst is less regenerated than the second catalyst by atleast a threshold amount.
 7. The method of claim 5, wherein theselecting further includes, selecting the first group of cylinders fordeactivation for a threshold number of consecutive deactivation cycleswhere the first catalyst is less regenerated than the second catalyst;and after the threshold number of consecutive deactivation cycles,selecting the second group of cylinders for deactivation even if thefirst catalyst is less regenerated than the second catalyst.
 8. Themethod of claim 5, wherein the selecting further includes, selecting thesecond group of cylinders to be deactivated when the second catalyst isless regenerated than the first catalyst.
 9. The method of claim 8,wherein the selecting further includes, selecting the second group ofcylinders to be deactivated when the second catalyst is less regeneratedthan the first catalyst by at least a threshold amount.
 10. The methodof claim 8, wherein the selecting further includes, selecting the secondgroup of cylinders for deactivation for a threshold number ofconsecutive deactivation cycles where the second catalyst is lessregenerated than the first catalyst; and after the threshold number ofconsecutive deactivation cycles, selecting the first group of cylinderseven if the second catalyst is less regenerated than the first catalyst.11. The method of claim 8, wherein the selecting further includes,selecting a group of cylinders that was not previously deactivated whenthe first and second catalysts are both regenerated by at least thefirst threshold amount.
 12. The method of claim 8, wherein the selectingis further based on a temperature of the first catalyst relative to thesecond catalyst.
 13. The method of claim 12, wherein the selecting basedon the temperature includes, selecting the first group of cylinders tobe deactivated when the temperature of the first catalyst is lower thana temperature of the second catalyst, even if the second catalyst isless regenerated than the first catalyst; and selecting the second groupof cylinders to be deactivated when the temperature of the secondcatalyst is lower than the temperature of the first catalyst, even ifthe first catalyst is less regenerated than the second catalyst.
 14. Amethod of operating an engine including a first group of cylinderscoupled to a first catalyst and a second group of cylinders coupled to asecond catalyst, comprising, selectively deactivating the first group ofcylinders on a first deactivation cycle; and on a following deactivationcycle; during a first mode including a temperature of the secondcatalyst being lower than the first catalyst and the first catalystbeing less regenerated than the second catalyst, selectivelydeactivating the second group of cylinders, and during a second modeincluding a temperature of the first catalyst being lower than thesecond catalyst and the second catalyst being less regenerated than thefirst catalyst, selectively deactivating the first group of cylinders.15. The method of claim 14, wherein the first mode includes each of thefirst catalyst and the second catalyst being regenerated by a thresholdamount.
 16. The method of claim 14, wherein deactivating the first groupof cylinders includes selectively disabling fuel injectors whilecontinuing valve operation of the first group of cylinders, and whereindeactivating the second group of cylinders includes selectivelydisabling fuel injectors while continuing valve operation of the secondgroup of cylinders.
 17. An engine system, comprising: a boosted engine;a first group of cylinders with selectively deactivatable fuelinjectors; a second group of cylinders with selectively deactivatablefuel injectors; a first catalyst coupled downstream of the first groupof cylinders; a second catalyst coupled downstream of the second groupof cylinders; a first set of oxygen sensors for estimating a firstair-to-fuel ratio differential across the first catalyst; a second setof oxygen sensors for estimating a second air-to-fuel ratio differentialacross the second catalyst; and a control system with non-transitorycomputer readable instructions for, selecting either the first group orthe second group of cylinders for deactivation based at least on aregeneration state of the first catalyst relative to the secondcatalyst, including based on whether the first and second catalysts areboth regenerated by at least a first threshold, and a difference betweentheir regeneration states is less than a second threshold, the selectingfurther including selecting cylinders based on a deactivation order whenboth catalysts are regenerated by the first threshold, and thedifference between their regeneration states is less than the secondthreshold.
 18. The system of claim 17 further comprising a firsttemperature sensor coupled to the first catalyst and a secondtemperature sensor coupled to the second catalyst, wherein the selectingis further based on a temperature of the first catalyst relative to thesecond catalyst.
 19. The system of claim 18, wherein the regenerationstate of the first catalyst is based on the first air-to-fuel ratiodifferential and the regeneration state of the second catalyst is basedon the second air-to-fuel ratio differential.
 20. The system of claim19, wherein the selecting includes, when the first catalyst isregenerated less than the second catalyst, selecting the first group ofcylinders for deactivation; when the second catalyst is regenerated lessthan the first catalyst, selecting the second group of cylinders fordeactivation; when the difference between regeneration states of thefirst and second catalysts are less than the second threshold, selectingthe first group of cylinders for deactivation if the temperature of thefirst catalyst is lower than the temperature of the second catalyst, andselecting the second group of cylinders for deactivation if thetemperature of the second catalyst is lower than the temperature of thefirst catalyst.